Method of seismic surveying



April 29, 1952 R. P. GREEN 2,594,767

METHOD OF SEISMIC SURVEYING Original Filed Nov. 1, 1943 mm F'LTER MAGNETIC TAPE I RECORDING HEAD SOURCE OF SEISMIC WAVES ADJUSTABLE SEISMIC OSCILLOGRAPHL SCREEN FILTER I 2 I I STATIONARY MIRRoR 9 I l E 1 l RO ARY MIRRoR 30 27 l l J SOURCE OF LIGHT g; X FIG. 2 INVENTOR.

ROBERT F. GREEN y My Wam A TTORNEYS Patented Apr. 29, 1952 METHOD or s PATENT F SEISMIC SURVEYING RobertiP. Green, Tulsa, Okla assignor to. Engineering Laboratories, Inc., Tulsa, Okla., a corporation of Oklahoma Original application November 1,1943, SerialNo. 508,590. Divided and this application February 8, 1946, Serial No. 646,341

8 Claims. (Cl. 175183) The present invention relates generally to the art of seismic surveying and more particularly to an improved method of recording and analyz ing information obtained by the seismic re-. flection method. This applicationvis a division of conending application Serial No. 508,59 0;'filed November 1, 1943, nowabandoned.

In general, the seismic reflection method of subsurface surveying. comprises, initiating an impulse somewhere within the earths crust and recording the resultant movement at a point-more or less distant from the: point of originofthe impulse, in. such manner that the time elapsing between the instant of the impulse and the ensuing recorded movementmay be determined. This method deals particularly with the seismic waves that are reflected from the underlying beds, as distinguished from refracted waves and other disturbances whichwill also be picked up by the recording device. It is of particular importance to consider the fact that the success of seismic surveying by the reflection method depends upon the ability of the observer to differentiate between the reflected energy and other energy arriving at the"Geo phone, since the results depend esseritially"upon the accuracy of.

determining the instant of arrival ofthe reflected waves at the Geophonefi It has been found that in many areas the reflected waves are completely masked by refracted waves,

ground roll disturbances, and various other;

sources of seismic noise.

In order to separate the reflected waves from the undesirable disturbances, advantage is taken of the frequency distribution of the various movements; that is, the frequencies of the reflected:

waves generally differ from the frequencies of refracted waves and other disturbances present. In order to produce legible seismic records that adapt themselves easily for geological interpretation, use has been made in the past of v It is common knowledge to those skilled in the 1 art that "the frequency band of the reflected energy is difle-rent in various physical locations. In certain sections of the United States, for example along the coast of the Gulf of Mexico, the

reflected energy generally covers a-bandof frequencies having maximum energy in the region of 35 cycles. In Mid-Continent areas, the reflected energy band of frequencies is shifted to a higher valuewith the middle of the band being in the region of 50 cycles, and in some cases substantially higher. This band of frequencies, however,'regardless of the frequency of maximum energy, is in the nature of a continuous frequency band and not of separate frequenc bands.

In prior .seismic recording systems a simple filter arrangement having fixed electrical characteristics has been generally used. Such a system will not serve well in all sections of the country as it will not permit emphasis on the particular frequency band which contains the'mostinformation in the various regions. In order to obtain full advantage of the energy in the reflected waves in such a system, it is necessary to adjust the response ofv the filtering equipment to the particular band of frequencies furnishing the most information'in a particular area. The response of the filter must be maximum in the region of 35 cycles for Gulf Coast work, and adjustable for higher frequencies for work in Mid- Continent area's. Inorder to determine the best I filtering adjustments in a given territory several preliminary tests are necessary. Suchv tests are .very expensive and-not .completely satisfactory because'each test requires a separate shot with only one record being obtained from each shot,

and if theflltering is found to be incorrect, ad-

ed and overcome.

It is a further object of this invention to provide an improved method of seismic surveying in which all the impulse energy picked up is initially recorded and the recording is thereafter reproduced and the various frequency bands thereof containing the most information are selected by visual trace inspection and then rerecorded. I

' A -feature of this invention is the recording of the earth vibrations produced by a seismic disturbance 'phonographicallyon a suitable recording medium and thereafter reproducing the recording into electrical signals under various filtering conditions and visually observing the traces under the varying conditions prior to rerecording.

A further feature of this invention is the provision of a system utilizing a variable speed motor for emphasizing the desired frequencies of a recorded seismic record.

Further objects, features, and advantages of the present invention will be apparent from the following description and reference to the ac companying drawings, in which:

Fig. 1 shows the general arrangement embodying my invention, including a means for producing a disturbance inthe earth, picking up vibrations caused thereby, and phonographically recording these vibrations, and;

Fig. 2 shows schematically a system for reproducing the recording and modifying the reproduction thereof.

Referring now more particularly to Fig. 1, elastic waves are generated by producing a .disturbance in the earth as by detonating. an explosive charge at the shot point Ill. This charge may consist of any suitable detonating material, such as dynamite, in quantities depending upon the particular conditions in the area. being .explored. For the purpose of picking up theyibrations caused by the. charge, a Geophone II is buried in the ground. .at a convenient distance from the shot point ID. This Geophone may be of any desired construction, such as themoving, coil type, carbon button type, or piezoelectric type, and mayfhave a resonant frequency of any desired number of cycles ,per second;

Waves emanating from the shot point Ill-arein the form of complex wave trains, some of which travel directly to the 'Geophone" .II

through the surface of the earth while others ,7

are transmitted downward to theinterfaces of the substratum, from which they are reflected and detected by the.Geophone. Various detailed methods of accomplishingthis. result are well-known to those skilledin the art and will not be discussed here. The elastic waves generated by the charge H! are translatedinto electrical currents by meansof the Ge'ophone H and amplified in the amplifier-l2. For the purpose of eliminating currents of certain frequencies which do'not contain useful information, the filter i3 is provided for transmitting, mainly, the frequency range which includes the contribution due to the reflected energy. The frequency range transmitted by the filter is,-however, sufiiciently wide to cover the frequency ranges of reflected energy in different sections of the country, as previously described. In order to record the currents transmitted by the filter IS a magnetic recorder is provided. This recorder comprises a magnetic recording head-I5 which receives energy from the filter and produces a record thereof upon a segment of a discontinuous magnetizable element in the form of a magnetic tape [9 running between supply and take up reels l6 and H, which are driven in a conventional manner. Magnetic recorders of this type are commercially available andare familiar to those skilled in the art.

It is apparent, therefore, that by means of the arrangement illustrated in Fig. 1, a permanent and phonographically reproducible record of the earth vibrationsis formed upon a segmentof the that a maximum amount of information-m 'ayabe:

obtained with regard to the particular area which ment of the magnetizable element [9 upon which the vibrations are recorded is formed into an endlessor continuous loop by cutting this segmentv from. the supply spool and suitably connecting the ends of the segment together. The

magnetic tape is continuously moved through or past a reproducing element in the form of a magnetic reproducing head 20, which includes aneIectro-magnet placed close to the magnetic tape. The reproducing head translates the rec- 0rd into electrical .signalscorresponding to the vibrations. recorded. thereon. The electrical .signalsthus .developedby the reproducing head 20 arefed, into an amplifier 2| and then transmitted through an. adjustable filter 22 to an oscillograph. 23,. where. the ,seismographic trace is visually reproduced. The endless magnetic tape is set into continuous motion by means of reels l8 drivemby a variable speed motor 9. A variable resistorunit-8 is provided inthe electrical power supplycircuit for the .motor 9 to permit'varying .the speediof the motor. .fl heioscillographfi includes a field magnet 24 'and'almovable coil' 2.5 which carries a mirror 26. Light'from an incandescent lamp 2.1'i's projected uponv .themirror 26 for reflection upon a fixed. mirror 29. The light reflected by the mirror .29 .is subsequently received on a multi sidedmirror 3B which, is rotated by a motor 3!. The. light reflected by themirror 30 is received upon. a transluscentscreen32 which. is concentric with the axis of the. mirror. $3.0. Rotation of the mirror 30 provides the necessary time coordinates to. the representation on the screen and .the movementof the coil, 25 and mirror 26 producedby the signaljcurrents transmitted byfllter 22 causes the beam impinging on the. screen 32 to deflect itself. perpendicularly to the timeaxis.

.Such oscillographkdevices are well-known in the art and are described, for-example, in Electrical Measurements by F. A. Laws, McGraweHill Book Company, New York, New York, 1917, pages 636 to .637.

The-magnetic tape 19 and the revolving mirror 3,!) are rotated synchronously with respect to each .other, thus producing. on the; screen .32 a

non-persistent visualrepresentation of the. sig nalsrecordeduponthe-tape l9. Becauseof. the

' slight luminous .retentivity of thescreen 32, the

particular area, the adjustable filter 22 is provided in the reproducing system which may be adjustedv to establish variousfiltering conditions.

.Forsseach set-of characteristicsgof the;fi1terz22 a different visual representation of the record will be presented upon the screen 32. Thus as the band pass characteristic of the filter is changed in a predetermined manner a number of visual representations canbe presented upon the screen 32 in succession, enabling an observer to determine the adjustments of the filter which give the best information. The selected signal frequency band may thereafter be permanently recorded, using conventional recording facilities, to provide the maximum amount of interpretive information without requiring additional shooting or rechecking in the field.

After original tests have been made in the particular area, information obtained as to the adjustment of the filter 22 providing the best information, can be used in designing a filter I 3 for the recording system which will give maximum response-to the frequencies of importance in that particular area. However, even in such cases the adjustable filter 22 will be of value in providing further selection to provide the best possible information.

It is apparent from an examination of the system of Fig. 2 that the-band pass characteristic of the adjustable filter 22 depends, in a great measure, on the speed of the motor 9. If the speed of the motor 9 is such that the magnetic the record, then the frequencies reproduced will be correspondingly lower and the effectiverange of the filter is changed. Because of this relationship between the speed of the motor 9 and the characteristics of the filter 22, it is possible.

to obtain selective filtering by using a filter having fixed constants and varying the speed of the motor 9. In many instances this method is superior to the method of varying the constants of the filter, since by varying the speed of the motor more simple operation and greater flexibility are obtained.

From the foregoing explanation it will be understood that in practicing the present improved method, the desired information from seismic surveying is obtained by using only one explosive charge and recording all vibrations picked up by the "Geophone which would be of any possible value in interpretation of the record in any geographic area. Thereafter the recording is repeatedly reproduced and the particular frequency band orv bands furnishing the most information in a particular area are selected by visually observing a series of modified reproductions of the record. After the frequenc bands furnishing the most information have been determined a permanent recording can be made of these bands for further study and interpretation.

While one embodiment of the invention has been described, it will be understood that various modifications may be made therein which are within the true spirit and scope ,of the invention as defined in the appended claims.

I claim:

1. The method of analyzing a phonographic record of seismic vibrations which comprises, repeatedly translating said record into electrical signals, translating those signals which are within a predetermined frequency band into a nonpersistent visual reproduction, and changing at will the band of frequencies which are reproduced, whereby the entire range of recorded vibrations may be observed to determine the frequency band of the record containing desired information.

2. In seismic prospecting, the method of analyzing a seismic record obtained by creating a disturbance in the earth at one point and phonographically recording the vibrations resulting from the disturbance at a point removed from said one point which comprises repeatedly reproducing said seismic record to develop electrical signals corresponding to said vibrations, translating the developed signals within a. predetermined frequency band into an instantaneous visual reproduction, and changing the band of signal frequency reproduction, whereby the various visual reproductions may be observed to determine the frequency band containing desired information. I

3. In seismic prospecting, the method of analyzing a seismic record obtained by creating a disturbance in the earth at one point and phonographically recording the vibrations resulting from the disturbance at a point removed from said one point which comprises, repeatedly reproducing said seismic record to develop electrical signals corresponding to said vibrations, adjustably filtering the developed signals, translating the filtered signals into an instantaneous visual reproduction, and changing the filtering to transmit signals in various predetermined frequency bands, whereby the various visual reproductions may be observed to determine the frequency band containingrdesiredinformation.

4. In seismic prospecting, the method of analyzing a seismic record obtained by creating a disturbance in the earth at one point and phonographically recording the vibrations resulting from the disturbance at a point removed from said one point which comprises, repeatedly translating all of said recorded vibrations into electrical signals by a variable speed reproducing unit, passing a band of said signals through a filter adapted to transmit a limited band of frequencies, translating said passed signals into a nonpersistent visual trace, and varying the speed of said reproducing unit to vary the frequency of said signals so that the signals representing different portions of the frequency range covered by said recorded vibrations are passed by said filter, whereby said traces may be observed to determine the setting of said variable speed reproducing unit which will result in production of a trace furnishing desired information.

5. In seismic prospecting, the method of analyzing a seismic record obtained by creating a disturbance in the earth at one point and magnetically recording the vibrations resulting from the disturbance at a point removed from said one point upon a segment of a discontinuous magnetizable element which comprises, forming said segment of said element into a continuous loop for repeated reproduction of the recorded vibrations, repeatedly reproducing as electrical signals the vibrations recorded upon said segment of said element, selecting on a frequency basis different bands of said signals during repeated reproductions of said vibrations, and visually reproducing the selected signals.

6. In seismic prospecting the method of analyzing a seismic record obtained by creating a disturbance in the earth at one point and magnetically recording the vibrations resulting from the disturbance at a. point removed from said one point upon a segment of a discontinuous magizabie element wh h ccmpri es, formin s segment of said element into a continuous loop for repeated reproduction of the recorded vibrations, repeatedly reproducing as electrical signals the vibrations recorded upon said segment of said element, selecting on a frequency basis different bands of said signals during repeated reproductions of said vibrations, and reproducing in nonpersistent visual form the selected signals, whereby the entire range of recorded vibrations may be scanned without permanent recording to determine the particular band of vibration frequencies containing desired information.

7. In seismic prospecting, the method of analyzing a seismic record obtained by creating a disturbance in the earth at one point and magnetically recording the vibrations resulting from the disturbance at a point removed from said one point upon a segment of a discontinuous magnetizable element which comprises, forming said segment of said element into a continuous loop for repeated reproduction of the recorded vibrations, continuously moving saidlooped segment of said element past a reproducing element to repeatedly reproduce as electrical signal the vibrations recorded upon said segment of said element, filtering said electrical signals to select therefrom only a band of the signals, visually reproducing the selected signals, and changing the speed of movement of said looped segment of said magnetizable element past said reproducing element, thereby to change the vibrations corresponding to the band of signals filtered and visually reproduced.

8. In seismic prospecting, the method of analyzing a seismic record obtained by creating a disturbance in the earth at one point and magnetically recording the vibrations resulting from the disturbance at a point removed from said one point upon a segment of a discontinuous magnetizable element which comprises, forming said segment of said element into a continuous loop for repeated reproduction of the recorded vibrations, continuously moving said looped segment of said element past a reproducing element to repeatedly reproduce as electrical signals the vibrations recorded upon said segment ofsaid element, filtering said electrical signals to select therefrom only a band of the signals, reproducing in non-persistent visual formthe selected signals, and changing the speed of movement of said looped segment of said magnetizable element past said reproducing element, thereby to change the band of signals filtered and visually reproduced and thus provide for scanning of the entire range of recorded vibrations without permanent recording to determine the particular band of vibration frequencies containing desired information.

ROBERT P. GREEN.

REFERENCES CITED The following references are of record in the file of this patent:

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